Electronic
Instrumentation
European PhD – 2009
Transducers and Signal Conditioning
Horácio Fernandes
Useful Signal
When converting a signal to a quantity, it
is only useful if its representation is kept
unchanged within a knew error
 Signal conditioning and transmission is
very important in applied physics

Electronic Instrumentation, PhD 2009
Signal paths

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
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Preserve signal quality
DAS less demanding
Preserve and adjust dynamic bandwidth
Resize operational limits
 Offset
 Amplitude
 Bandwith



Linearization
Galvanic isolation
Buffering
Electronic Instrumentation, PhD 2009
Sensors and Transducers

Transducers
 Device
capable of changing one form of energy into
another



Active – External power supply
Passive – Internal source (self-generating)
Sensors
 Changing
of a characteristic in an electric circuit (R;
L, C);
 Generate an output signal proportional to the stimulus
Electronic Instrumentation, PhD 2009
Sensores
Related energy
Example
Comment
Mechanic
Flow-meter
Strain gauge
Pulse counter
Momentum transfer
Thermal
Thermocouple
Thermal radiation
Junction voltage
Infrared sensor
Electromagnetic
Antenna
Space electromagnetic power converted to
electric signals
Magnetic
Hall sensor
MDH Probes
Voltage derived from Hall Effect
Induced EMF
Chemic
pH sensor
Ionic concentration
Nuclear
Ionization chamber
Scintillators
Current generation induced by free charges
Indirect light proportional to brehmstrallung
Electronic Instrumentation, PhD 2009
Sensors
Transducer Principles

Resistive
 Strain
gauges: Force measurements (W.
Bridges)
 Temperature: RTDs, termistors
 Light: photoelectric cells and photodiodes
 Position: potentiometers as dividers, grids
Electronic Instrumentation, PhD 2009
Sensors
Transducer Principles

Capacitive
 Movement
 Dielectric constant
 Geometric configuration
 Cell chargers

Inductive
–Differential
Transformer
 Hall Effect
 Motors as generator
 LVDT
Electronic Instrumentation, PhD 2009
Sensors Selection

Scale: limiting extremes (Worst Case)
 multiples

Threshold
 Least

sensors for scale spanning
detected variation (resolution)
Behavior
 Temporal
response
 Dynamic response
 Accuracy and resolution
 Stress (consistency)
 Reproducibility and hysteresis

Price
Electronic Instrumentation, PhD 2009
Sensors Operation

Environment
 Dirty
 Pollution
 Extreme
Temperatures
 Water presence and moist
 Chemical corrosion: solvers, acids e bases
 Environmental protection
 Susceptibility: eletric/explosion/chash
Electronic Instrumentation, PhD 2009
Sensors Operation

Human use
 Radiation
 Corrosion/Chemicals
manipulation
 Immersion
 Erosion/Vibrations
 Explosion
 Electric
Interference (EMI- high impedance,
low current)
Electronic Instrumentation, PhD 2009
Sensors Operation

Power
 Circuit
Charger (photocell)
 Excitation source (noise)
Signal Conditioning
 Physics size

Electronic Instrumentation, PhD 2009
Calibration
Measurement Error – Comparison
standard should be more exact than
sensor resolution
 Calibration table – Calibration curve

 Physic
model
 Static and dynamic calibration
 Bandwidth
 Impulsive response
Electronic Instrumentation, PhD 2009
Linearization

Transfer function
errors
 Non-linearity




Sensor
Electronics
Signal path
Compensation
 Non-linear
electronic
circuit
 Piecewise interpolation
Electronic Instrumentation, PhD 2009
Buffering

Source/Input isolation
 Impedance
adaptation
Maximum feed power
 Voltage signal


Transducer output
 Preserve

signal
Next stage charge circuit
Electronic Instrumentation, PhD 2009
Meters and bridges
Differential mode
 Common mode

Electronic Instrumentation, PhD 2009
Wheatstone Bridge

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Potentiometer divider
Zero Measurement
CMR>100 dB
Sensibility
Thermal immunity
Ra  Rb
Rx 
Rc
Electronic Instrumentation, PhD 2009
Wheatstone Bridge
Application
Electronic Instrumentation, PhD 2009
Kelvin bridge
Very low resistors
(<1R)
 Double terminals

Electronic Instrumentation, PhD 2009
Maxwell Bridge
Electronic Instrumentation, PhD 2009
Bridges Configurations
Electronic Instrumentation, PhD 2009
Bridges circuits
AC generators
 Current sources
 OPAMPs applications

Electronic Instrumentation, PhD 2009
Bridge noise immunity
Pick-up noise
 Cable resistance
 Signal Bandwidth
 3-wire connection

Electronic Instrumentation, PhD 2009
Noise reduction

…If noise blocking fails in the origin…
…Nightmare begins!
Electronic Instrumentation, PhD 2009
What can we filter?
Signal sampling: analog goes digital at
what rate?
 Nyquist criteria: fs>2fmax
 Low-pass filters (cutoff -40 dB)
 Guard-band
 Sampling band: [fs-fmax, fs+fmax]

Electronic Instrumentation, PhD 2009
Useful Storage Bandwith

Pratical figures
 USB=fs/2.5

Sin Interpolation
 USB=fs/10

Linear Interpolation
 USB=fs/25

No Interpolation
 USB=fs/4.6

Digital correction (factor 1.6) x (1/0.35)
Electronic Instrumentation, PhD 2009
Aliasing
Electronic Instrumentation, PhD 2009
Perceptual aliasing
Electronic Instrumentation, PhD 2009
Image aliasing
Electronic Instrumentation, PhD 2009
Filters
Pass-band
 Cut-off
 Stop-band
 Ripple
 Order
 Phase and amplitude
characteristics

Electronic Instrumentation, PhD 2009
Common Filters


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
Lowpass
Highpass
Passband
Notch
Digital filtering (made possible with fast ADCs)
 Advantages:
High order, cutoff frequency, complex
transforms
 Signal correlation
Electronic Instrumentation, PhD 2009
W1: TENSILE.1.STRAINRATE
W2: Linear Regression
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W3: 13th Order Polynomial Fit
100 120 140 160 180 200 220 240 260 280
W4: Both Fits and Data
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W5: Linear Fit Residuals
8
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W6: Polynomial Fit Residuals
6
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Electronic Instrumentation, PhD 2009
Other Techniques

Periodic signals
 Lock-in

amplifiers
Cross correlation
 Boxcar
integration
 Multichannel mean
Overlap of periodic signals
 S/N~N1/2

Electronic Instrumentation, PhD 2009
Other Techniques

Pulsed signals
 Constant
Fraction
discriminator
Electronic Instrumentation, PhD 2009
Transimpedance amplifier

Allow very low current
sources detection, ex:
photodiodes
 Tomography
 Spectrometers
 Line


radiation filters
Charge measurements, ex:
ion beam
High bandwidth
Electronic Instrumentation, PhD 2009
I(V) probes

Current detection
 Ground
loop
 Safety - galvanic isolation
 Sweep waveforms – capacitive coupling and
distortion
 Fast sweeping – plasma limit operation
Electronic Instrumentation, PhD 2009